Printed circuit board and method for producing a printed circuit board

ABSTRACT

A printed circuit board for an electronic circuit, especially for the ultra-high frequencies located in the GHz range that comprises at least one conductor layer, which is arranged on top of an insulating layer and which is flatly joined to said insulating layer. Improved mechanical, thermal and electrical properties are attained by virtue of the fact that the insulating layer is a thin glass layer.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a printed circuit board, a method forproducing such a printed circuit board, and a layer composite materialfor such a printed circuit board.

2. Prior Art

In present-day electronic circuit technology concerning, in particular,computer technology and the communication and processing of data incommunications technology, development is tending toward an ever greaterintegration and packing density and toward ever higher clockfrequencies, which are normally already in the GHz range. Under thesecircumstances, increasing importance is being attached to the printedcircuit boards or printed circuits in the construction of functionallyreliable and, at the same time, cost-effective circuits. On the onehand, such a printed circuit board, in particular for relatively largeseries, must be simple and cost-effective to produce and process (cut,drill, populate, etc.). On the other hand, the printed circuit boardmust have a sufficient thermal and mechanical stability and the bestpossible thermal conductivity, because a considerable amount of heat isgenerally to be expected owing to the high integration density and highfrequencies during operation. Furthermore, it is desirable for theprinted circuit board to be adapted to the ubiquitously usedsemiconductor components (made of Si or else GaAs) with regard tothermal expansion, because this enables direct mounting of thesemiconductor chips on the printed circuit board (Direct Chip AttachDCA) with all its advantages.

However, the dielectric properties of the insulation material usedwithin the printed circuit board are particularly important with regardto the high frequencies. Thus, a printed circuit board used in theextremely high frequency range should have an insulating layer betweenthe conductor layers with the lowest possible (relative) permittivityε_(r) and a small dielectric loss factor tanδ in order to keep down thelosses that increase with the frequency.

Finally, at the high clock rates and with very fine conductor tracks, itis becoming more and more important for the printed circuit boardmaterial to be distinguished by a high degree of homogeneity in thedielectric and a high uniformity in the external dimensions (smallthickness fluctuations, etc.) because otherwise, in adjacent regions ofthe printed circuit board, undesirable propagation time differences areproduced during the signal propagation and impair the functionality ofthe circuit constructed therewith.

All the requirements presented are satisfied only poorly or not at allby conventional epoxide-based substrate materials. Therefore, variousproposals have already been made in the past in respect of using, forsingle or multilayer printed circuit boards, insulating intermediatelayers made of a sintered glass ceramic which, at the same time, havegood dielectric properties and are adapted to GaAs circuits, forexample, in terms of their thermal expansion coefficient (see e.g. U.S.Pat. No. 6,017,642). However, such sintered glass ceramic substrates arecomplicated to produce and, as ceramic plates, have only a limitedmechanical strength, so that they allow the realization of, inparticular, thin single-layer printed circuit boards only withdifficulty.

Another proposal relates to the use of a “glass paper”—produced fromglass fibers—as dielectric for printed circuit boards (JP-A-9208252).Although such a fiber material should be less at risk of fracturecompared with the glass ceramic, the irregular fibrous structure of thematerial results in a local inhomogeneity in the dielectric properties,which can lead to the abovementioned propagation time problems with highline densities and at high frequencies.

Finally, a whole class of substrate materials for printed circuit boardsis known which comprise fluoropolymers filled with additives (ceramicparticles, glass fibers) (see e.g. U.S. Pat. No. 5,149,590). Althoughsuch materials, which are commercially available for example from the UScompany Rogers Corp. under the designations RT/DUROID 5870–5880 andRO3000, exhibit relatively good and homogeneous dielectric propertiesand are therefore well suited to extremely high frequency applications,such a material is comparatively expensive on account of the complicatedproduction and, moreover, has an unfavorable thermal expansioncoefficient which is significantly higher than that of silicon.

SUMMARY OF THE INVENTION

Therefore, it is an object of the invention to provide a printed circuitboard which avoids the disadvantages of known printed circuit boardsand, in conjunction with comparatively simple and cost-effectiveproduction, is distinguished by very good mechanical and thermalproperties, has very good dielectric properties and can thus be used upto extremely high frequencies, and is optimally adapted to thecustomarily used Si semiconductor chips in terms of its thermalexpansion. Furthermore, it is an object of the invention to specify amethod for producing such a printed circuit board.

The heart of the invention consists in providing a thin glass layer, asis known principally from the technology of liquid crystal displays(LCDs), as dielectric in the printed circuit board below the at leastone conductor layer comprising conductor tracks. Even with a smallthickness, such a thin glass layer has a good mechanical stability, hasvery advantageous dielectric and thermal properties and is distinguishedby a high optical quality, which is manifested in particular in a highhomogeneity of the material and a high degree of planarity with smallevenness and thickness deviations. The use of a thin glass layer asdielectric makes it possible to satisfy all the requirements which aremade of printed circuit boards for electronic circuits with a highintegration density and extremely high frequencies.

A preferred refinement of the invention is characterized in that arespective conductor layer is arranged on both sides of the thin glasslayer and connected to the thin glass layer in planar fashion. In thiscase, one conductor layer or both conductor layers may be structured,i.e. comprise individual conductor tracks. Such a thin glass layerprovided with a conductor layer on both sides has the advantage over theone which is coated on one side that an electronic circuit realizedtherewith is provided with an unambiguously defined volume which ispredominantly filled with the thin glass dielectric and hascorrespondingly favorable properties.

Preferably, the thin glass layer is pulled from the melt and comprises amodified borosilicate glass, the thin glass layer having a thickness inthe range from a few μm to a few mm, preferably in the range between 30μm and 1.1 mm, and having, at 1 MHz, a relative permittivity ε_(r) ofbetween 6 and 7, preferably of about 6.2, and a dielectric loss factortanδ of about 9×10⁻⁴ and having, for temperatures of between 20 and 300°C., a thermal expansion coefficient α₂₀₋₃₀₀ of between 4×10⁻⁶K⁻¹ and8×10⁻⁶K⁻¹, preferably of about 4.5×10⁻⁶K⁻¹.

In an advantageous manner, the conductor layers in each case comprise ametal foil, preferably a Cu metal foil, have a thickness of between 5and 50 μm, preferably of 18 or 35 μm, and are adhesively bonded to thethin glass layers in each case by means of a connecting layer, theconnecting layers essentially comprising a resin. Such resin-coated Cufoils (Resin Coated Foil or RCF) are known from the technology of HDIcircuits, i.e. printed circuit boards with high integration densities.

Preferably, the connecting layers are additionally provided with asilane as adhesion promoter. Furthermore, it is advantageous if thesurfaces of the thin glass layers are pretreated in order to improve theadhesion.

In the simplest case, the printed circuit board comprises an individualthin glass layer which is adhesively bonded to metal foils on bothsides. Given a small thickness of the thin glass layer, this results ina thin, flexible printed circuit board having very good electrical andthermal properties.

However, it is also conceivable for the printed circuit board tocomprise, in a stack one above the other, a plurality of thin glasslayers adhesively bonded to metal foils and thus to form a multilayerprinted circuit board which may also be provided with plated-throughholes in the customary manner.

Equally, it is conceivable for the printed circuit board to comprise,besides a thin glass layer, at least one further insulating plate madeof a different insulating material. The combination of conductorlayer(s) and thin glass layer can thus advantageously be integrated intoa printed circuit board of a conventional type.

Further embodiments emerge from the dependent claims.

BRIEF EXPLANATION OF THE FIGURES

The invention will be explained in more detail below using exemplaryembodiments in connection with the drawing, in which

FIG. 1 shows, in a sectional illustration, the construction of anunstructured printed circuit board in accordance with a first preferredexemplary embodiment of the invention;

FIG. 2 shows the printed circuit board from FIG. 1 after the (two-sided)structuring of the conductor layers and with a semiconductor chip which,by way of example, is applied directly by means of the DCA method;

FIG. 3 shows, in a sectional illustration, the construction of a(structured) printed circuit board in accordance with a second preferredexemplary embodiment of the invention;

FIG. 4 shows, in a sectional illustration, the construction of a(structured) multilayer printed circuit board in accordance with a thirdpreferred exemplary embodiment of the invention;

FIGS. 5 a and 5 b show a method for producing a layer composite materialfor a printed circuit board according to FIG. 1 in accordance with apreferred exemplary embodiment of the invention; and

FIGS. 6 a –6 f show a method for producing a printed circuit boardaccording to FIG. 3 in accordance with another preferred exemplaryembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be explained below using examples in which a thinglass layer is provided with a structured or unstructured conductorlayer in each case on both sides. However, it is also conceivable, inprinciple, within the scope of the invention to provide a conductorlayer only on one side of the thin glass layer. The construction and themethod for producing a printed circuit board or a layer compositematerial with such a thin glass layer “occupied” on one side emergeanalogously from the explanations below.

FIG. 1 represents, in a sectional illustration, the construction of anunstructured printed circuit board in accordance with a first preferredexemplary embodiment of the invention. In this case, the printed circuitboard 10 comprises an individual thin glass layer 13, a conductor layer11 and 15 respectively being arranged on the two sides of said thinglass layer. The conductor layers 11, 15 are formed by Cu metal foilswhich are adhesively bonded to the thin glass layer 13 in planar fashionby means of an adhesive connecting layer 12 and 14, respectively.

Thin glass layers of the kind provided in the context of the inventionand used to achieve the advantages according to the invention are knownfrom the prior art (see e.g. EP-A1-0 972 632 or DE-A1-198 10 325). Theyhave thicknesses of between about 30 μm and 2 mm, are pulled from themelt, and are distinguished by a good mechanical and optical quality andexcellent thermal and electrical properties. Such thin glass layers areproduced commercially on a relatively large scale for electroopticaldisplays (e.g. LCDs) and their price is therefore comparativelyfavorable.

Suitable thin glass layers or thin glasses are produced and offered bythe company Schott Glas, Mainz (Del.) under the type designations AF 45and D 263 T.

The thin glass AF 45 is a modified borosilicate glass with a highproportion of BaO and Al₂O₃. It is normally in sizes of up to 440 mm×360mm and is distinguished by the following properties:

AF 45 Thickness (standard): 50 μm to 1900 μm Transformation point T_(g):662° C. Thermal expansion coefficient a_(20–300): 4.5 × 10⁻⁶K⁻¹ Rel.permittivity ε_(r) (at 1 MHz) 6.2 Dielectric loss factor tanδ:   9 ×10⁻⁴

The thin glass D 263 T is a borosilicate glass produced from very purestarting materials. It is likewise available in sizes up to 440 mm×360mm and is distinguished by the following properties:

D 263 T Thickness (standard): 30 μm to 1100 μm Transformation pointT_(g): 557° C. Thermal expansion coefficient a_(20–300): 7.2 × 10⁻⁶K⁻¹Rel. permittivity ε_(r) (at 1 MHz): 6.7 Dielectric loss factor tanδ:  61× 10⁻⁴

The two thin glass layers or thin glasses are particularly well suitedto use in a printed circuit board according to the invention.

The production of a suitable layer composite material for the(unstructured) printed circuit board 10 in accordance with FIG. 1 ispreferably effected in the manner illustrated in FIG. 5: what is used asa starting point is a thin glass layer 13, whose surfaces are firstlyfreed of adhereing moisture (water) and subsequently pretreated—e.g. bymeans of a corona discharge or the like—in order to obtain a good,permanent adhesion during the subsequent adhesive bonding. In order toproduce the conductor layers 11, 15 on the thin glass layer, use is madeof resin-coated metal foils (Resin Coated Foils RCFs) 28, 29 made of Cu,a connecting layer 12 and 14 respectively being applied to the adhesivebonding side thereof (FIG. 5 a). The connecting layers 12, 14 contain aresin which, expediently, is partly cured or prereacted. Suchresin-coated Cu metal foils are known from the technology of large scaleintegrated (HDI) circuits.

In the context of the invention, particularly suitable resin-coatedfoils are offered for example by the company Isola AG(Del.) under thedesignation ISOFOIL 160 and RCC. The (ISOFOIL 160) foils have athickness of the copper foils of. 18 μm or 35 μm, for example, and areprovided with a 75 μm resin layer which is prereacted (so-called B stagecoating).

The resin-coated foils 28, 29 are packed, with the additional use of asilane layer for adhesion promotion (in this respect, see e.g. U.S. Pat.No. 5,149,590), with the pretreated central thin glass layer 13 to forma stack. The stack is then adhesively bonded under the action of heatand pressure (FIG. 5 b). A layer composite material or an (unstructured)printed circuit board 10 in accordance with FIG. 1 is obtained as aresult. The conductor layers 11, 15 can then optionally be structuredusing known methods from printed circuit board production, in order toproduce as required specific conductor tracks (or conductor areas) 16,17 (FIG. 2). By way of example, SMD devices or the like can then besoldered or conductively bonded onto the structured printed circuitboard 10 in accordance with FIG. 2. A particularly advantageous type ofmounting is made possible by virtue of the thermal expansioncoefficients of the thin glass layer 13 which are adapted to thecustomary semiconductor materials: thus, in accordance with FIG. 2, asemiconductor chip 18 provided with corresponding connection contacts 19can be mounted directly on the printed circuit board 10 (so-calledDirect Chip Attach DCA). As a result of this, even higher packingdensities can be achieved in conjunction with simplified mounting andhigh reliability.

In addition to the simple printed circuit board 10 from FIGS. 1, 2 and 5which comprises only one thin glass layer 13 and two conductor layers 11and 15 and can therefore be made particularly thin and flexible, it isalso possible to realize in an analogous manner multilayer printedcircuit boards 30 which comprise, in a stack one above the other, aplurality of thin glass layers 13, 13′ and 13″ with conductor layers 34and 35 lying in between and outer conductor layers 31, 38 (FIG. 4). Inthis case too, each conductor layer is provided with a correspondingconnecting layer 32, 33, 36, 37 which adhesively bonds the conductorlayer to the respective thin glass layer. In this case, the innerconductor layers 34, 35 are expediently embedded in an associatedconnecting layer 33 and 36, respectively. It goes without saying thatsuch a multilayer printed circuit board 30 may also be provided withplated-through holes (not shown in FIG. 4) which are produced in amanner known per se and connect conductor tracks in different conductorlayers to one another.

Another possibility within the scope of the invention consists inintegrating a thin glass layer that is provided with a conductor layeron one side or on both sides into a printed circuit board of aconventional type. Thus, the example illustrated in FIG. 3 shows aprinted circuit board 20 in which a central thin glass layer 13 withconductor layers 23, 24 and conductor tracks 27, 27′ bonded on bothsides is arranged between two insulating plates 21, 26 of a conventionaltype (e.g. based on epoxy resin or polytetrafluoroethylene) andadhesively bonded to said insulating plates. Connecting layers 22, 25once again serve here for adhesive bonding, the conductor layers 23, 24being embedded in said connecting layers.

The production of such a “hybrid” printed circuit board 20 withconventional insulating plates and thin glass layers is illustrated invarious steps in FIGS. 6( a)–(f): the starting point is one of theinsulating plates, namely the insulating plate 21, onto which a firstconductor layer 23 is bonded in a conventional manner by means of afirst connecting layer 39 and is then structured (FIG. 6 a). The firstconnecting layer 39 is then filled up whilst covering the firstconductor layer to form the final connecting layer 22 (FIG. 6 b). Thestructure 21, 22, 23 thus prepared is thereupon adhesively bonded on oneside to a correspondingly pretreated thin glass layer 13 (FIG. 6 c). Afurther structure 24, 25, 26 can then be bonded onto the free side ofthe thin glass layer 13 (FIGS. 6 e,f), which structure comprises theother insulating plate 26 provided with a second conductor layer 24. Inthis case, the second conductor layer 24 is bonded onto the insulatingplate 26 by means of a second connecting layer 40, and subsequentlystructured (FIG. 6 d), and the second connecting layer 40 is then filledup to form the final connecting layer 25 (FIG. 6 e). In this exemplaryembodiment too, it is conceivable to use resin-coated Cu metal foils toproduce the conductor layers 23 and 24.

Overall, the invention yields a printed circuit board which isdistinguished by the following properties and advantages:

-   -   The printed circuit boards can be made very thin without losses        in mechanical stability    -   A direct mounting of semiconductor chips is possible by virtue        of the adaptation of the thermal expansion coefficients of the        central dielectric (thin glass) to the customary semiconductor        materials    -   The optical quality, homogeneity and uniformity in the        dimensions of the thin glass material avoid disturbing        influences on the signal propagation, in particular at high        frequencies    -   The comparatively good thermal conductivity of the thin glass        layer facilitates the dissipation of heat and thus allows higher        integration densities    -   The small relative permittivity and the small dielectric loss        factor of the thin glass material enable the circuit arranged on        the printed circuit board to have very high frequencies    -   The thin glass layers are commercially available in outstanding        quality and at favorable prices from series production and        therefore limit the costs of the printed circuit boards    -   The thin glass layers can be worked and processed well in the        context of printed circuit board production; in particular,        known methods of printed circuit board production can be used        without difficulty    -   Multilayer printed circuit boards can be produced without        difficulty, which boards can be made even more compact on        account of the small thickness of the thin glass layers    -   Even finer conductor track structures are possible on account of        the high degree of homogeneity and uniformity of the dielectric.

The layer composite material produced by the method according to theinvention is particularly well suited as starting material for theprinted circuit board. However, it is also conceivable to use thismaterial in other applications.

LIST OF REFERENCE SYMBOLS

-   10, 20, 30 Printed circuit board-   11, 15 Conductor layer (Cu metal foil)-   12, 14 Connecting layer-   13, 13′, 13″ Thin glass layer-   16, 17 Conductor track-   18 Semiconductor chip-   19 Connection contact (semiconductor chip)-   21, 26 Insulating plate (e.g. epoxy)-   22, 25 Connecting layer-   23, 24 Conductor layer (Cu metal foil)-   27, 27′ Conductor track-   28, 29 Resin-coated metal foil (Cu)-   31, 34, 35, 38 Conductor track-   32, 33, 36, 37 Connecting layer-   39, 40 Connecting layer

1. A method of forming a printed circuit board comprising: (a) providinga first layer of borosilicate glass; (b) laminating first and secondmetal foils to first and second sides of the layer of borosilicateglass; (c) forming a first circuit pattern having at least one conductortrack in the first metal foil; and (d) operatively mounting asemiconductor chip to the first circuit pattern.
 2. The method of claim1, wherein each metal foil has a thickness no greater than 50 μm.
 3. Themethod of claim 1, further including: forming a second circuit patternhaving at least one conductor track in the second metal foil; andconnecting at least one conductor track of the first circuit pattern andat least one conductor track of the second circuit pattern via one ormore plated through holes formed through the first layer of borosilicate glass.
 4. The method of claim 1, further including: (e) forminga second circuit pattern having at least one conductor track in thesecond metal foil; (f) laminating a second layer of borosilicate glassto the second circuit pattern; (g) laminating a third metal foil to aside of the second layer of borosilicate glass opposite the secondcircuit pattern; and (h) forming a third circuit pattern having at leastone conductor track in the third metal foil.
 5. The method of claim 4,further including: connecting at least one conductor track of the secondcircuit pattern and at least one conductor track of the third circuitpattern via one or more plated through holes formed through the secondlayer of borosilicate glass.
 6. The method of claim 4, wherein: thesecond layer of borosilicate glass is laminated to the second circuitpattern via a connecting layer; and the second circuit pattern isembedded in the connecting layer used to laminate the second layer ofborosilicate glass to the second circuit pattern.
 7. The method of claim4, further including operatively mounting another semiconductor chip tothe third circuit pattern.
 8. The method of claim 4, further including:(i) laminating a third layer of borosilicate glass to the third circuitpattern; (j) laminating a fourth metal foil to a side of the third layerof borosilicate glass opposite the third circuit pattern; (k) forming afourth circuit pattern in the fourth metal foil; and (l) operativelymounting another semiconductor chip to the fourth circuit pattern. 9.The method of claim 8, wherein: the third layer of borosilicate glass islaminated to the third circuit pattern via a connecting layer; and thethird circuit pattern is embedded in the connecting layer used tolaminate the third layer of borosilicate glass to the third circuitpattern.
 10. The method of claim 8, wherein each metal foil is laminatedto its layer of borosilicate glass via a corresponding connecting layer.11. The method of claim 10, wherein the connecting layer comprises aresin and a silane.
 12. A printed circuit board comprising: a firstlayer of borosilicate glass; first and second metal foils laminated tofirst and second sides of the layer of borosilicate glass, wherein afirst circuit pattern having at least one conductor track is definedfrom the first metal foil; and a semiconductor chip operatively mountedto the first circuit pattern.
 13. The printed circuit board of claim 12,wherein: a second circuit pattern having at least one conductor track isdefined from the second metal foil; and the first layer of borosilicateglass includes one or more plated through holes connecting at least oneconductor track of each of the first and second circuit patterns to eachother.
 14. The printed circuit board of claim 13, further including: asecond layer of borosilicate glass laminated to the second circuitpattern; a third metal foil laminated to a side of the second layer ofborosilicate glass opposite the second circuit pattern, wherein a thirdcircuit pattern having at least one conductor track is defined from thethird metal foil; and the second layer of borosilicate glass includesone or more plated trough holes connecting at least one conductor trackof each of the second and third circuit patterns to each other.
 15. Theprinted circuit board of claim 14, wherein: the second layer ofborosilicate glass is laminated to the second circuit pattern via aconnecting layer; and the second circuit pattern is embedded in theconnecting layer.
 16. The printed circuit board of claim 14, whereineach metal foil is laminated to its side of borosilicate glass via acorresponding connecting layer.
 17. The printed circuit board of claim16, wherein each connecting layer comprises a resin, and a silane. 18.The printed circuit board of claim 12, wherein: the first layer ofborosilicate glass has a thickness no greater than 1.1 mm; each metalfoil is laminated to the first layer of borosilicate glass via aconnecting layer having a thickness no greater than 20 μm; and eachmetal foil has a thickness no greater than 50 μm.
 19. A printed circuitboard comprising: a first metal layer; a first layer of borosilicateglass having a first side laminated to a side of the first metal layer;a second metal layer laminated to a second side of the first layer ofborosilicate glass; a second layer of borosilicate glass having a firstside laminated to a side of the second metal layer opposite the firstlayer of borosilicate glass; and a third metal layer laminated to asecond side of the second layer of borosilicate glass, wherein: at leasttwo of the metal layers have circuit patterns defined therein; eachcircuit pattern has at least one conductor track; conductor tracks ofthe metal layers having circuit patterns defined therein areelectrically connected by one or more plated through holes formed in oneor more of the layers of borosilicate glass; and at least onesemiconductor chip is operatively mounted to one of the first and thirdcircuit patterns.
 20. The printed circuit board of claim 19, whereineach metal foil is laminated to its side of borosilicate glass via acorresponding connecting layer.
 21. The printed circuit board of claim20, wherein each connecting layer comprises a resin and a silane.